1. Field of the Invention
This invention relates to a roller member having a conductive (or semiconductive) elastic layer, which is preferably usable as a charging or transfer roller, a developing roller, a transport roller, a fixing roller, an intermediate transfer roller or the like used in image forming apparatus utilizing an electrophotographic process, such as electrophotographic apparatus, and also relates to a process for its manufacture.
2. Related Background Art
FIG. 1 diagrammatically illustrates the constitution of an electrophotographic apparatus having a contact charging means and a contact transfer means. Reference numeral 1 denotes a drum type photosensitive member comprising a conductive support made of, e.g., aluminum, and provided on its peripheral surface a photoconductive layer. Reference numeral 2 denotes a charging roller which is in contact with this photosensitive member and charges the surface of the photosensitive member to a stated potential.
This charging roller is kept in pressure contact with the photosensitive member 1 at a stated pressure by a pressure contact means (not shown) such as springs, and is followingly rotated as the photosensitive member 1 is rotated in the direction of an arrow A. Also, a DC and AC (or only DC) bias is applied to this charging roller to charge the photosensitive member 1 to a stated potential. That is, in order to obtain good copied images, it is necessary for the charging roller 2 to be in the state of uniform contact with the photosensitive member 1 and have conductivity. The surface of the photosensitive member 1 charged to a stated potential by the charging roller 2 is exposed to imagewise modulated exposure light 3 emitted from an exposure means (not shown) such as a laser or an an LED (light-emitting diode), whereupon an electrostatic latent image corresponding to the intended image information is formed on the photosensitive member 1.
Subsequently, the electrostatic latent image formed is rendered visible as a toner image by a developing means 4 having a developing roller 401. This toner image on the surface of the photosensitive member 1 is transferred to a transfer material 6 on its surface side by charging the transfer material 6 on the back thereof to a polarity reverse to that of the toner by means of a transfer roller 5. The transfer material 6 to which the toner image has been transferred is separated from the photosensitive member 1, and then the toner image on the transfer material 6 is fixed to the transfer material 6 by a fixing member 7 by the action of heat and pressure. Also, from the surface of the photosensitive member 1 from which the toner image has been transferred, deposits such as residual toner at the time of transfer are removed by a cleaning member 8 to make the surface clean. Thus, the photosensitive member 1 is repeatedly used for image formation. In FIG. 1, reference numeral 9 denotes a toner.
Roller members such as the charging roller 2, the transfer roller 5 and the developing roller 401 are each constituted of at least a mandrel supported rotatably on its both ends and an elastic layer having a conductivity (semiconductivity) of about 1×103 to 1×1010 Ω·cm as volume resistivity, provided on the peripheral surface of the mandrel. Such an elastic layer is produced using a rubber composition containing a conductive rubber or a rubber with conductive particles dispersed therein, and a vulcanizing agent and optionally containing a vulcanization accelerator and so forth, and by extruding the rubber composition into a roller in such a way that it covers the peripheral surface of the mandrel, and thereafter vulcanizing the rubber. However, air bubbles (hereinafter “voids”) may be produced in the elastic layer as a result of the evaporation of water incorporated in the rubber or rubber composition, caused by the heat at the time of vulcanization.
A roller member to the elastic layer surface of which such voids have come exposed to make the elastic layer have concavities on its surface may cause faulty images when it is used in the electrophotographic process. For example, where such a roller is used as the charging roller, the charging roller may cause faulty charging at the concavities to cause faulty images as a result of it. Such faulty images caused by voids may occur even where the elastic layer surface has been subjected to modification treatment such as irradiation with ultraviolet light, irradiation with electron rays or impregnation with a surface treatment solution. Also, especially where a surface coat layer is formed on the elastic layer by coating, even if the voids standing exposed to the elastic layer surface are microscopic, their presence on the elastic layer surface may cause large crater-like defects on the surface coat layer which have grown from the voids as nuclei. Such a phenomenon has been seen. Then, such voids may remarkably occur especially in using an ion-conductive rubber having a polar group in the molecule. This is considered due to the fact that the polar group tends to adsorb water molecules.
To solve such problems, a method is proposed in which a rubber is compounded with a dehydrating agent which removes water content of unvulcanized rubbers, as exemplified by calcium oxide (see, e.g., Japanese Patent Application Laid-Open No. H09-297454). However, the dehydrating agent such as calcium oxide has a poor dispersibility in rubbers, and also the compounding of rubber with the dehydrating agent may make the elastic layer have a high hardness.